This invention relates to improved electrodes for unitized electron guns used in television cathode ray tubes, and is particularly concerned with means and method for compensating for beam spot distortion introduced by certain cathode ray tube components.
Optimum resolution is a much desired and sought after characteristic in television picture tubes. Resolution is largely a function of the size and symmetry of the beam spots projected by the electron gun of the picture tube, or guns in plural-beam tubes. Beam spots are desirably relatively small, symmetrically round, and uniform in size at all points on the picture screen. Achievement of these ideals has been difficult because of the many factors which influence the configuration of beam spots. As a result of these factors, a beam spot that is symmetrical at the center of the picture imaging field can become distorted at the periphery of the field, as will be shown.
The key factors which influence beam spot configuration and symmetry of picture tubes having three-beam unitized electron guns include electron gun design, cathode ray tube screen potential, magnitude of beam current, the "throw" distance from the electron gun to a given point on the screen, and the convergence means. Of these, distortion of the beam spot configuration induced by the electron gun mechanical design and the magnetic convergence means are most relevant to problems resolved by the present invention.
The term commonly applied to the phenomenon of center-screen-to-side distortion is "deflection defocusing." The effect of deflection defocusing when a self-converging yoke is employed for beam deflection is shown diagrammatically by FIG. 1, wherein a cathode ray tube faceplate 10 is shown as having a luminescent screen 12 indicated schematically by the peripheral dashed line. A beam spot 14 is shown as being symmetrically round when at its landing point at the center 16 of screen 12. At the periphery of the screen 12, however, the beam spot is shown to be everywhere elliptized, as indicated by beam spots 18, with the major axes of the ellipses directed toward the screen center 16. (The effect shown applies to a gun designed such that the focus track will not allow operation of the beam in a core/halo mode as deflection takes place.)
A major cause of deflection defocusing is attributable to the design of the yoke. Asymmetrical center spots are largely attributable to gun design. In a gun having lens electrodes made up of continuous tubes, or "barrels," there is negligible distortion of the beam.
This effect is shown schematically by FIG. 2 wherein an electron beam 20, as seen from a viewpoint concentric with its axis, is indicated as passing through a continuous cylindrical lens element 22 (beam-passing tube). Beam 20 is subject to the influence of electrostatic forces which are radially equal, as indicated by equal-length arrows 24. This electrode structure leads to the projection of a symmetrically round beam spot 26 on the screen.
In in-line guns structured without beam-passing tubes, however, the beams are subject to asymmetric influences resulting in distortion of the beam spots. This effect is shown schematically by FIG. 3 wherein a rectangular metallic electrode 28 is indicated as having three beams 30, 32 and 34 passing therethrough. With regard to beam 32 (normally the beam of the "green" gun), the beam, unlike beam 20 of FIG. 2, is subject to an unequal division of forces. The metal of electrode 28 in the shape of a rectangle that forms an apparent lens similarly rectangular that subjects the beam to "pulling" forces, indicated by the outwardly directed arrows lying in the minor axis of rectangular electrode 28, and at the same time, to "pushing" forces, indicated by the inwardly directed arrows lying in the major axis. These forces lead to beam spots having either of the elliptical configurations shown by beam spots 36 and 38. Whether the major axis of the elliptical beam spot lies in the horizontal plane or in the vertical plane depends upon factors such as whether or not the immediate lens action is positive or negative, and whether or not the beam is being accelerated or decelerated.
Beams 30 and 34, lying near the sides of electrode 28, are subject to different force vectors; the forces are even more unbalanced because of the propinquity of the beams to one of the ends of electrode 28 and in remoteness from the opposite end. As a result, the beam spots will be substantially asymmetric but basically elliptized as indicated by spots 36 and 38. But rather than the very symmetrical ellipses shown, the spots will be more of a configuration resembling a tear drop, as indicated by beam spot 40 produced by beam 30, and beam spot 42 produced by beam 34.
The components that provide both static and dynamic magnetic convergence can similarly distort beam spots at the periphery of the screen. Static convergence is commonly provided by quadrupolar and sextipolar magnetic fields, and dynamic convergence by the "self-converging" yoke now in common use in conjunction with in-line guns in color television picture tubes.
Static convergence of the electron beams, and the adjustment of the "color purity" of reproduced images is provided by an assembly of circumferential magnets located around the neck of the cathode ray tube in the areal region of the main focus lens of the electron gun. Such an assembly is illustrated by FIG. 4, shown in association with a color cathode ray tube display of the self-converged type. Briefly, the illustrated system comprises a tube envelope 44 on the neck 46 of which is mounted a magnetic yoke assembly 48, the color purity/static convergence assembly 50, and a printed circuit board assembly 52. The forward part of the envelope 44 is broken open to show the CRT faceplate 54, a phosphor screen 56 on the inner surface of the faceplate 54, a shadow mask 58 spaced from the screen, and three coplanar "in-line" electron beams 60, 62 and 64 generated by an electron gun assembly (not shown) in the neck 46 of the tube. Also shown on the tube is a bundle of yoke leads 68 and a high-voltage connector 70 through which the anode voltage is brought through the tube envelope for application to the screen 56. A base for the tube is shown at 72.
To effect static convergence of the beams, and to adjust the "color purity" of the reproduced images, the purity/static convergence assembly 50 comprises three components: a bi-polar purity adjustment component 74, and quadrupolar and sextipolar static convergence adjustment components 76 and 78. A ring-gear drive arrangement is provided for the convergence adjustment components 74, 76 and 78 so that they can be driven in the desired rotational directions when an associated drive gear is turned. As related pairs of multipolar magnets are contra-rotated, their respective fields either align or cancel, permitting a resultant magnetic field of any desired strength to be obtained. Thus, by appropriate control of the relative rotational positions of the magnets, the three in-line electron beams can be shifted in unison from side-to-side to effect purity control, and, by means of components 76 and 78 which comprise the quadrupolar and sextipolar magnets, each beam can be moved relative to the other to effect convergence of the beams on the screen. The purity/static convergence assembly 50 is disclosed in detail and fully claimed in U.S. Pat. No. 4,050,041, assigned to the assignee of the present application.
While providing the benefits of static convergence and purity control, the quadrupolar adjustment component 76 of assembly 50 exerts a deleterious effect on the configuration of the beams passing through its field of influence. The effect is illustrated schematically in FIG. 5 wherein the electron beams 60, 62 and 64 of an in-line gun are shown as being surrounded by the quadrupolar field 80, indicated by the dashed lines, of the quadrupolar static convergent adjustment component 76 shown by FIG. 4. The polarities of the quadrupolar field 80 are indicated by the positive and negative symbols. The effect of the quadrupolar field 80 is shown by the adjacent series of beam spots, wherein beam spot 62a of beam 62 is shown as being elliptically distorted. The distortion of off-center beams 60 and 64 as indicated by the beam spots 60a and 64a wherein the spots are shown to be irregular ellipses skewed inwardly toward the center beam 62, also as a result of the distortive influence of quadrupolar field 80.
Another of the convergence components that produces beam spot distortion in deflection is the self-converging yoke field. Yoke assembly 48 (FIG. 4) is a hybrid type having toroidal-type deflection vertical coils and "saddle-type" horizontal deflecting coils. The yoke is of the self-converging type and contains windings which produce an astigmatic field component having the effect of maintaining the beams in convergence as they are swept across the screen. The astigmatic field component which self-converges the beams, however, undesirably introduces an astigmatic deflection defocusing of the beams when the beams are deflected off the tube axis. Since spherical aberration cannot be eliminated entirely, but only minimized, and since the yoke astigmatisms are necessary if self-convergence is to be achieved, it is difficult if not impossible to completely "design out" this deflection defocusing problem.
However, an acceptable compromise can be effected. Beam spot ellipticity at the screen periphery, wherein the major axis of the ellipse is in the horizontal plane, can be reduced and its effects alleviated by causing the beam spot at its landing point at the center of the screen to be elliptical in the vertical plane. As the beam is deflected, the same phenomenon that causes the undesired peripheral ellipticization works to make the center-ellipticized beam spot round at some distance between the screen center and its periphery, and relatively round at the screen periphery. The penalty of vertical beam spot ellipticity at the center of the screen is minimal in line-screen type cathode ray tubes in common use. Any loss in resolution at the center is more than compensated for by the greatly increased resolution in the non-central zones resulting from the relatively round beam spots.
Various approaches have been taken to reduce the real or apparent effects of deflection defocusing of the beams at the screen peripheries. One approach is described in U.S. Pat. No. 3,984,273. It involves the provision of vertically oriented elliptical apertures in the G2 electrode of the gun. The resulting beam spot shape is vertically elliptical at the screen center; that is, a shape that is orthogonal to the horizontal beam deflection defocusing produced by the factors described in the foregoing. Some compensation for deflection defocusing is attained; however, there are a number of drawbacks to this approach. It is believed for example that the amount and perhaps even the direction of the ellipticity induced in the beam changes as a function of beam current; hence this "elliptized aperture" approach is relatively ineffectual. Second, the use of such a gun is limited to a given design to conform to a particular cathode ray tube size and configuration. Also, it is well known that any gun having apertures which are not rounded is difficult and costly to assemble. Electron guns having round apertures are assembled by the well known technique of "mandrelling" the parts and thereafter conjoining the parts by the use of heat-softened glass rods. Electrodes having non-cylindrical apertures cannot be precisely aligned on rod-like mandrels designed for circular apertures. Another example of this approach is found in U.S. Pat. No. 3,881,136.
Another approach involves forming a round beam in the lower end of the gun, as is conventional. In the main focus lens of the gun, a quadrupolar astigmatic field component is formed which introduces a vertical elongation of the beams at the screen center. The vertical elongation attained at least partially compensates for deflection defocusing of the beams. This technique is employed in a non-standard color CRT display system in which three electron guns are arranged to share a common main focus lens. A dynamic quadrupolar magnetic field is established in the main lens which rounds out the beams. This system is described in an article titled "25-Inch 114 Degree Trinitron Color Picture Tube And Associated New Development," Sony Corporation, IEEE Spring Conference on BTR, June 10, 1974.
This latter-described system offers the advantage of producing no astigmatism in the beam when the yoke field is zero; that is, when the beams are in the center of the screen. It has the disadvantage, however, that in rounding out the beams at the edges of the screen, the beam spots are undesirably enlarged. It has been found to be necessary in such a system to use relatively costly dynamic focusing along with deflection defocusing compensation in order to minimize the spot enlargement at the screen edges. Dynamic focusing is normally not needed in modern-day color television receivers.
U.S. Pat. No. 4,086,513 to Evans, Jr. discloses a plural gun cathode ray tube having parallel plates, or "extensions," adjacent to grid apertures of a bipotential electron gun. The stated objective is to distort the electrostatic field formed by at least one of the two electrodes nearest the screen, forming noncircular electron beams to compensate for distortion of the beams in the magnetic deflecting field. In one embodiment, plates are positioned on opposite sides of each aperture and extend toward the screen from one of the focusing electrodes, resulting in vertical elongation of the undeflected beam spot. In another embodiment, horizontally oriented parallel slats or plates are attached to the inner wall of a cup-shaped accelerating and focusing anode to cause defocusing about the vertical axis passing through the electrode apertures. The result is also vertical elongation of the beam spot. Electrodes according to the Evans, Jr. disclosure are characterized by the creation of a circularly symmetrical field which is deliberately distorted by the extending plates or slats to obtain the desired beam spot ellipticity. Further, the gap between the electrodes is constant, and the extensions are common to the entire electrode. The drawback to the use of such extensions include the problems in production in producing such a complex structural addition, and, in the embodiment wherein the slats project into the gap, the problem of inter-electrode arcing induced by the extensions.